Message challenge response

ABSTRACT

A system and method are disclosed for registering a email sender for the purpose of sending an email message to an email receiver including receiving a challenge message wherein the challenge message includes a machine answerable question; processing the challenge message to determine that it is a challenge message sent for the purpose of authorizing delivery of the email message; analyzing the question to determine a valid response to the challenge message; and sending the valid response that includes the answer wherein a nontrivial amount of resources are required to send the valid response.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation and claims the prioritybenefit of U.S. patent application Ser. No. 14/226,598 filed Mar. 26,2014, which is a continuation and claims the priority benefit of U.S.patent application Ser. No. 13/787,693 filed Mar. 6, 2013, now U.S. Pat.No. 8,732,256, which is a continuation and claims the priority benefitof U.S. patent application Ser. No. 10/387,352 filed Mar. 11, 2003, nowU.S. Pat. No. 8,396,926, which is a continuation-in-part and claims thepriority benefit of U.S. patent application Ser. No. 10/197,393 filedJul. 16, 2002, now U.S. Pat. No. 8,924,484, the disclosures of which areincorporated herein by reference.

The present invention is related to U.S. patent application Ser. No.10/422,359 filed Apr. 23, 2003, now U.S. Pat. No. 7,539,726, thedisclosure of which is incorporated herein by reference.

The present invention is also related to U.S. patent application Ser.No. 11/927,523 filed Oct. 29, 2007, now U.S. Pat. No. 7,908,330, whichis a continuation and claims the priority benefit of U.S. patentapplication Ser. No. 10/387,352 filed Mar. 11, 2003, now U.S. Pat. No.8,396,926, which is a continuation-in-part and claims the prioritybenefit of U.S. patent application Ser. No. 10/197,393, now U.S. Pat.No. 8,924,484, filed Jul. 16, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electronic messages. Morespecifically, a technique for avoiding spam emails is described.

2. Description of the Related Art

Electronic messages have become an indispensable part of moderncommunication. Electronic messages such as email or instant messages arepopular because they are fast, easy, and have essentially no incrementalcost. Unfortunately, these advantages of electronic messages are alsoexploited by marketers who regularly send out unsolicited junk messages(also referred to as “spam”). Spam messages are a nuisance for users.They clog people's email box, waste system resources, often promotedistasteful subjects, and sometimes sponsor outright scams.

To reach a broad audience, the senders of spam (also referred to as“spammers”) usually generate a huge amount of messages using automatedcomputer programs. There are many existing techniques for filtering spammessages, some of which attempt to identify machine generated messagesand block them. Upon receiving a message, the recipient may issue achallenge that is difficult for a computer to answer but easy for aperson. For example, the challenge may include a picture of threeobjects, with the question “how many objects are in the picture?” Thesender needs to choose three from a number of available answers and sendthe answer to the recipient. This type of question, referred to as ahuman answerable question, is difficult for machines to answer correctlybut easy for human beings. The reply is then examined by the recipient'scomputer. The message is delivered to the recipient if the answer iscorrect, and processed as spam otherwise. This type ofchallenge/response scheme incurs significant overhead in for the senderof the email, making email no longer “free” and therefore more difficultfor the spammers.

The challenge/response scheme described above is effective for blockingspam messages, but has many drawbacks. For example, since the message isnot delivered until the challenge is answered, long delays may result ifthe sender is not available to answer the challenge right away. Also,the scheme is inconvenient for legitimate users that need to send outmessages to new addresses often, for example a business that has manynew clients. It would be desirable to have the benefit of thechallenge/response scheme for spam blocking purposes, but does notresult in long delays in message delivery or excessive burden for thesender.

In one exemplary embodiment of the present invention, a method ofregistering an electronic-mail (e-mail) sender for the purpose ofsending e-mail to a recipient is provided. The e-mail sender receives achallenge message in response to an e-mail message previously sent bythe e-mail sender, the challenge message including a machine answerablequestion. The challenge message is then processed to determine that themessage is a challenge message sent for the purpose of authorizingdelivery of e-mail messages. Processing the challenge message in thisregard includes identifying an indicator that the message is a challengemessage. The message is then parsed to locate the machine answerablequestion in the challenge message. The question is then analyzed todetermine a valid response to the challenge message. A nontrivial amountof resources are required to determine the valid response to thechallenge message. The response to the challenge (i.e., the validresponse) is then sent that includes an answer to the machine answerablequestion. Receipt of the response subsequently results in the e-mailsender being registered as a valid e-mail sender. As such, subsequente-mail messages sent by the e-mail sender will not require sending avalid response to a challenge message.

Another embodiment of the present invention includes a computer-readablestorage medium having embodied thereon a program. The program isexecutable by a computer processor to perform a method of registering anelectronic-mail (e-mail) sender for the purpose of sending e-mailmessages to an e-mail recipient.

A still further embodiment of the present invention discloses anelectronic-mail (e-mail) server configured to complete a registrationprocess for sending e-mail messages to art e-mail recipient. The e-mailserver includes a network interface for receiving a challenge message inresponse to an e-mail message sent by the e-mail server. The challengemessage is received over a communications network and includes a machineanswerable question. The server also includes memory configured to storecomputer-executable instructions for answering the challenge message anda processor configured to execute the instructions stored in memory.When executed, the instructions provide for processing the challengemessage to determine that the message is a challenge message for thepurpose of authorizing delivery of e-mail messages. Processing thechallenge message includes identifying an indicator that the message isa challenge message. The instructions also provide for parsing themessage to locate the machine answerable question in the challengemessage followed by analyzing the question to determine a valid responseto the challenge message. A nontrivial amount of resources are requiredto determine the valid response. Finally, the instructions provide forsending the valid response via the network interface and communicationsnetwork. The valid response includes the answer to the machineanswerable question and receipt of the valid response results in thee-mail server being registered as a valid e-mail server. As a result,subsequent e-mail messages sent by the e-mail server will not requiresending a valid response to a challenge message.

Another embodiment of the presently disclosed invention provides amethod of registering an electronic mail (e-mail) sender for the purposeof sending e-mail messages to an e-mail recipient. The e-mail senderreceives a challenge message in response to an e-mail message previouslysent by the e-mail sender, the challenge message including a machineanswerable question. The challenge message is then processed todetermine that the message is a challenge message sent for the purposeof authorizing delivery of e-mail messages. Processing the challengemessage in this regard includes identifying an indicator that themessage is a challenge message. The message is then parsed to locate themachine answerable question in the challenge message. The question isthen analyzed to determine a valid response to the challenge message. Anontrivial amount of resources are required to determine the validresponse to the challenge message. A response to the challenge messageis then sent to an auditor, which is a different entity than the e-mailrecipient. The auditor sends a report concerning the response to thee-mail recipient that identifies that the response includes an answer tothe Machine answerable question and that the e-mail sender should beregistered as a valid e-mail sender. As a result of being registered,subsequent e-mail messages sent by the e-mail sender will not requiresending a valid response to a challenge message to the auditor.

In yet another embodiment of the presently disclosed invention, a methodof classifying electronic-mail (e-mail) messages from an e-mail senderto an e-mail recipient is disclosed. Through this particular method, achallenge message is generated. The challenge message includes a machineanswerable question and for which a corresponding response may begenerated by a machine associated with the e-mail sender. Generating theresponse to the message includes a nontrivial amount of resources withrespect to processing the challenge message to determine that themessage is a challenge message sent for the purpose of classifyinge-mail messages including identification of an indicator that themessage is a challenge message; parsing the message to locate themachine answerable question in the challenge message; and analyzing thequestion to determine a valid response to the challenge message. Thechallenge message is then sent to the e-mail sender. A determination islater made as to whether a valid response to the challenge message hasbeen sent by the e-mail sender. A valid message results in the senderbeing classified as a valid sender and excepted from future challengemessages.

Another embodiment of the present invention describes acomputer-readable storage medium having embodied thereon a program. Theprogram is executable by a computer processor to perform a method ofclassifying electronic-mail (e-mail) messages from an e-mail sender toan e-mail recipient.

In a still further embodiment of the present invention, anelectronic-mail (e-mail) server configured to classify an e-mail messagefrom an e-mail sender to an e-mail recipient is provided. The serverincludes memory configured to store computer-executable instructions forclassifying an e-mail message and a processor configured to execute theinstructions stored in memory. When executed, the instructions cause thegeneration of a challenge message including a machine answerablequestion and for which a corresponding response may be generated by amachine associated with the e-mail sender. Generating the responserequires a nontrivial amount of resources with respect to processing thechallenge message to determine that the message is a challenge messagesent for the purpose of classifying e-mail messages includingidentification of an indicator that the message is a challenge message;parsing the message to locate the machine answerable question in thechallenge message; and analyzing the question to determine a validresponse to the challenge message. The message is then sent via anetwork interface configured to send the challenge message over acommunications network. A valid response to the challenge messageincludes the answer to the machine answerable question and results inthe e-mail sender being classified as a valid e-mail server and exceptedfrom future challenge messages.

Another embodiment of the present invention includes a method ofclassifying an electronic-mail (e-mail) message from a message sender toa message recipient. A challenge message including a machine answerablequestion is generated and for which a corresponding response may begenerated by a machine associated with the e-mail sender. Generating theresponse requires a nontrivial amount of resources with respect toprocessing the challenge message to determine that the message is achallenge message sent for the purpose of classifying e-mail messagesincluding identification of an indicator that the message is a challengemessage; parsing the message to locate the machine answerable questionin the challenge message; and analyzing the question to determine avalid response to the challenge message. The challenge message is sentto the e-mail sender and a response is received at an auditor, wherein,the auditor is a different entity than the message sender. The auditoris configured to send a report concerning the response to the messagerecipient that identifies that the response includes an answer to themachine answerable question and that the e-mail sender should beclassified as a valid e-mail sender whereby subsequent e-mail messagessent by the e-mail sender will not require sending a valid response to achallenge message to the auditor.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1 is a block diagram illustrating a challenge/response system thatincludes a third party.

FIG. 2 is a flowchart illustrating an exemplary challenge/responseprocess. The process begins when mail sender 100 sends a message to mailreceiver 102 (200).

FIG. 3 is a flowchart illustrating how mail sender 100 handles achallenge, according to one embodiment of the present invention.

FIG. 4 is a flowchart illustrating the audit process according to oneembodiment of the present invention.

DETAILED DESCRIPTION

It should be appreciated that the present invention can be implementedin numerous ways, including as a process, an apparatus, a system, or acomputer readable medium such as a computer readable storage medium or acomputer network wherein program instructions are sent over optical orelectronic communication links. It should be noted that the order of thesteps of disclosed processes may be altered within the scope of theinvention.

A detailed description of one or more preferred embodiments of theinvention is provided below along with accompanying figures thatillustrate by way of example the principles of the invention. While theinvention is described in connection with such embodiments, it should beunderstood that the invention is not limited to any embodiment. On thecontrary, the scope of the invention is limited only by the appendedclaims and the invention encompasses numerous alternatives,modifications and equivalents. For the purpose of example, numerousspecific details are set forth in the following description in order toprovide a thorough understanding of the present invention. The presentinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the present invention is notunnecessarily obscured.

An improved email registration system is disclosed. In thisspecification, email registration is used to refer to any process, task,or other requirement placed on a sender of an email (or the system thatsent the email) to allow the email to be delivered. One type ofregistration is challenge/response, as described above. Once thechallenge is sent and the response is correctly returned, the email issuccessfully registered so that it can be delivered. Other emails fromthe same sender or associated with the sender in some way may also beconcurrently or subsequently delivered as a result of the registration.

Several improvements to existing human answerable challenge/responseprocesses are disclosed. An auditor is included in thechallenge/response architecture that simplifies the response procedureand optionally monitors and approves responses. A machine answerablechallenge is used that is directed to a module on a mail sender that isconfigured to receive and answer the challenge. The challenge mayrequire a small but nontrivial amount of processing by the mail senderso that an entity sending millions of mails to addresses for which ithas not registered would be taxed by the process while a normal senderwould be relatively unburdened answering only challenges to registerwith new recipients. The machine answerable challenge may also include ahuman answerable element. In some embodiments, registration isaccomplished using a modified challenge wherein no response is requiredfor registration.

As used herein, an email sender may refer to any individual, device,organization or other entity that is associated with the sending of anemail. Such an entity may be identified in any manner that isappropriate for a given application. For example, the sender may beidentified by an email address, a domain name, an IP address or otheridentifier that identifies a mail server or other source of the email.When a sender is registered, the registration may be only valid for thespecific email that was challenged in some cases. More likely, thesender wilt be listed as a valid sender (“whitelisted”), or at least asa sender that can respond to challenges. Email from whitelisted sendersmay be delivered without further testing or challenges in some cases.The “sender” or entity that is whitelisted may include just the sourceemail address or the all addresses in the domain of the source emailaddress or any other relevant set of entities identified in any usefulmanner including by mail server or IP address. The registration of asender may be sufficient of itself to authorize delivery of an email orgroup of emails, or the registration may be a factor considered inclassifying the email along with other factors.

FIG. 1 is a block diagram illustrating a challenge/response system thatincludes a third party. Mail sender 100 and mail receiver 102 includesoftware and/or hardware capable of sending and receiving messages. Mailsender 100 and mail receiver 102 may be either clients or servers,depending on how a given mail system is configured. They may be standalone programs or systems, or plug ins that are parts of other programsor systems. A message is sent from mail sender 100 and is received bymail receiver 102 for an intended recipient. If mail receiver 102determines that the message may possibly be spam, it generates achallenge and sends the challenge to mail sender 100.

In some embodiments, mail sender 100 automatically processes thechallenge, generates a response, and sends the response to auditor 104.In some embodiments, mail sender 100 alternatively lets the usermanually answer the challenge, then generates a reply based on theanswer and sends the reply to auditor 104. The auditor is a third partyserver that independently verifies the reply and sends a report to mailreceiver 102. In some embodiments, the auditor resides on the samedevice as mail receiver 102, and is able to communicate the verificationresults to mail receiver 102.

The auditor provides several improvements to the challenge/responseprocess in various embodiments. First, the process of responding to achallenge is simplified in some embodiments by providing in thechallenge one or more links to a web server maintained by the auditor.For example, multiple links may be provided, each corresponding to apossible answer. Thus, only one action, selection of the correct link,is required of the sender to manually respond to a challenge. Inaddition, the auditor, because it sees all challenge/responses, candetect patterns in responses such as many responses coming from the samedomain and determine if a spammer is attempting to defeat the system. Ifsuch an event is detected, then the auditor can determine not toauthorize delivery in its report to mail receiver 102 or send additionalinformation along with its report to mail receiver 102 identifying themessage as probably spam to facilitate mail receiver 102 furtheranalyzing the message and making a correct classification decision. Itshould be noted that the report from the auditor can be used as a factorin the mail classification decision made by the mail receiver and thatthe auditor report does not necessarily solely determine theclassification.

In some embodiments, the challenge includes a machine answerablequestion that is more easily answered by a machine than a human. Forexample, the challenge may include a problem that requires extensivecomputation or searching to solve. Mail sender 100 is capable ofmonitoring incoming challenges and automatically answering the machineanswerable question. Thus, problems associated with having a personanswer the challenge questions are overcome. However, if spammers couldcopy the portion of the mail client that answers challenges and thenanswer challenges in an automated fashion without incurring any penalty,they could possibly defeat the challenge/response system.

The machine answerable challenge is designed such that to process andanswer the challenge, a nontrivial amount of resources is consumed. Anontrivial amount of resource is defined as an amount of resources thatis small in a single instance, but when many instances (on the order of1000) of such nontrivial amounts are added together, the collectiveamount of resource is large enough to incur significant costs. Anontrivial amount of system resource may have a small impact on thesystem in a single instance but may impact the system's operationssignificantly when many instances are added together. As computersystems increase in speed, the amount of processing that would berequired to be nontrivial would increase as well.

The nontrivial amount of resources may be adjusted as desired in aspecific system, but generally is greater than the minimum amount ofresource necessary to generate a properly formatted reply message. Theimpact of consuming a nontrivial amount of resource for processing achallenge, or even several challenges, is relatively small and does notaffect a regular user very much. However, for a spammer who sends outmillions of spam messages, the number of challenges he receives would beon the order of thousands and therefore the collective resource consumedin handling the challenges would be significant. By associating a smallcost with every response to a challenge, the improved challenge/responsesystem makes it more difficult for the spammers to successfully deliverlarge amounts of email.

Many different types of resources are applicable. For example, thechallenge question may be processing intensive, and the resourceconsumed is CPU cycles. In one embodiment, the challenge question askshow many prime factors are in a very large integer. The algorithm usedto calculate the number of prime factor is computationally intensive,and takes between a fraction of a second to a few minutes to run. Whilethe machine is answering a challenge and performing the computations, itmay slow down a bit, but the overall performance of the system does notappear to change very much to the sender who's using the computer. Onthe other hand, if a spammer receives thousands of challenge messagesback, the amount of CPU cycles needed to process the challenges wouldsignificantly slow down the spammer's computer system, thus hamperingthe spammer's operations. In different embodiments, the machineanswerable question consumes various types of resource, such as memoryand system resources.

One advantage of the number of prime Factors question is that it lendsitself to a multiple choice answer. The correct answer may be, forexample, three, and that may also be the answer to an alternativequestion that a user may answer relating to the number of objects in agraphic image. Other computationally intensive challenges are used, suchas asking for two prime factors of a large number. The amount ofresources may be adjusting the size of the number that must be factored.

In some embodiments, the resource is money. Small amounts of money (alsoreferred to as micropayments) are required in the response process. Inone embodiment, the sender's response to a challenge authorizes theauditor to collect a small micropayment that would be insignificantunless a very large number of responses are sent. The system may bestructured so that the sender has some money deposited in an escrowaccount, and a small amount of money is deducted every time he answers achallenge. Alternatively, the response may authorize payment and thesender is billed if a significant payment accrues.

In some embodiments, the resource is some type of rights such aslicensing rights. For example, a legal copy of the mail client may onlybe able to answer a limited number of challenges within a period oftime. Since it is unlikely that spammers would actually acquire a largenumber of legal licenses for the mail client, they would not be able tolegitimately send out large quantities of spam.

It should be noted that the various embodiments of nontrivial responsecosts may be used with the architecture shown in FIG. 1 or in anarchitecture that does not include the third party auditor. For example,if factoring a large number is used, the mail receiver may multiply twoprimes, store the factors along with an identifier for the email, andwait to determine whether the proper response is received from the mailsender.

FIG. 2 is a flowchart illustrating an exemplary challenge/responseprocess. The process begins when mail sender 100 sends a message to mailreceiver 102 (200). Upon receiving the message, mail receiver 102 sendsa challenge to mail sender 100 (202). The challenge includes thequestion or problem, as well as a mail identifier so that if a responseis received, it can be matched with the correct mail message and thecorresponding challenge. Meanwhile, the message is kept in a temporarylocation, such as a temporary folder.

Mail sender 100 includes special software or hardware that enables it toanswer the challenge question automatically. Mail sender 100 receivesthe challenge (204), processes the challenge to determine the correctresponse, and sends a reply that includes the response and the mailmessage identifier to the auditor (206). In different embodiments, theauditor may either be on a separate device or on the same device as mailreceiver 102. In the embodiment shown, the auditor is third partysoftware that resides on a server separate from mail receiver 102.

The auditor receives the reply from mail sender 100 and performs anauditing process on the reply (208). The auditing processing of the mailclient by the auditor varies for different embodiments and is describedin detail later. The auditor generates a report based on the processingresults and sends the report to mail receiver 102 (210). In someembodiments, the report is an email message. The report may also be anhttp request or may be sent using any appropriate communication linkbetween the auditor process, and the email delivery process. The reportmay include the status of the sender's address and/or informationregarding whether the answer to the challenge question is correct. Insome embodiments, the auditor leaves the answer verification up to mailreceiver 102 and includes the answer received from mail sender 100 inits report to mail receiver 102. Alternatively, the auditor may evaluatethe response from the mail sender and send either an authorization forthe mail message or a confirmation that the mail message is spam.

Based on the report from the auditor, mail receiver 102 processes theoriginal message (212). In some embodiments, the processing includesverification of the reply received from mail sender 100. In someembodiments, the processing also includes classifying the message. Forexample, if the message is not from a known spammer and the answer tothe challenge is correct, the original message is moved from thetemporary location that stores the message to the recipient's inbox;otherwise, the message is considered spam and is deleted. Mail receiver102 then further processes the report from the auditor, if necessary,and deletes the report (214).

FIG. 3 is a flowchart illustrating how mail sender 100 handles achallenge, according to one embodiment of the present invention. Theprocess begins when mail sender 100 receives a message from mailreceiver 102 (300). It is then determined whether the message is achallenge (302), using techniques such as analyzing the header and/orthe body of the message.

If the message is determined not to be a challenge, the processing stops(303); otherwise, the message is further processed. The message isparsed (304), and then analyzed to locate the question (306). In oneembodiment, the challenge is formatted as a Hypertext Markup Language(HTML) document, and an attribute marked with an HTML tag <alt> is usedto store the question. The text following the <alt> tag is a largeinteger, and the question is to determine the number of prime factors inthis large integer. The mail client then proceeds to answer the question(308). For questions such as the prime factor question, the mail clientuses built in functions to automatically perform such computation.

Since computing the prime factors of a sufficiently large number is acomputationally intensive process, it takes the mail client a nontrivialamount of time to arrive at the correct answer. A reply that includesthe answer is then formatted (310) and sent to the auditor (312). Inthis embodiment, to format the reply, a universal resource locator (URL)of the auditor is found in the challenge. An HTTP request that includesthe auditor URL, the message identifier of the challenge, the answer andthe address of the original sender is formatted and sent to the auditor.In other embodiments, other appropriate protocols are used to format theresponse.

FIG. 4 is a flowchart illustrating the audit process according to oneembodiment of the present invention. In this embodiment, the auditor isa third party that resides on a server separate from the mail clients.The process begins when mail sender 100 sends its reply to the auditor(400). The auditor receives the reply (402), and performs processingincluding parsing the reply (404) and locating information within thereply (406).

In some embodiments, the auditor uses the information to performoptional verification steps (408 and 410). The message identifier (ID)of the original message is checked by the auditor to verify that thereply is not fake (408). The auditor verifies the answer to the questionand determines whether the answer is correct (410).

Based on the processed results, the auditor creates a report and sendsthe report to mail receiver 102. In one embodiment, the report is anHTTP request that includes the message identifier, the original sender,and the answer to the challenge. In other embodiments, the report givesan instruction to classify the message or a probability that the messageshould be classified in a certain way. The report may also providefurther information about the message and the results of thechallenge/response process.

Specialized software code or a hardware component is used by mail sender100 to automatically answer the challenge question. However, a givenmessage sender may not have a mail client capable of automaticallyanswering the challenge question. To make the challenge answerable evenwhen the mail client does not have the answering feature, in someembodiments, a human answerable question, such as previously mentionedquestion concerning the number of objects in a picture, is also includedin the challenge. As noted above, the correct answer to the humananswerable question may match the correct answer to the machineanswerable question.

A sender's reply is generated based on the answer selected by thesender. The reply may be sent to the mail receiver directly, or sent toa third party auditor that verifies the reply and generates a report tothe mail sender to aid in classifying the message. In one embodiment,the challenge is formatted as an HTML document, with answers embedded asURL links. Selecting the correct answer causes an HTTP request to besent, where the HTTP request may include information about the answer,the message identifier of the original message, information about theoriginal sender, or combinations thereof. Embedding the answers as linksto a web server maintained by the auditor simplifies the responseprocess for the mail senders and allows the response to be generated bya single “click” or selection.

Many spammers send so many messages that it is not practical for them toreceive responses at all of the email addresses that they havegenerated. If an email challenge is sent to in response to such a spammessage, that challenge will often bounce. This fact is the basis of anull challenge used in one embodiment. The null challenge simply sends areply to the mail sender's email acknowledging receipt of the email orincluding some other message, but without inviting a response. If thereply from the mail receiver is delivered successfully and no errormessage is received within a certain period of time, then the mailreceiver considers the challenge to have been successful. Thus, the nullchallenge simply pings the mail sender to make sure the from address inthe message is a valid address where mail may be sent. The nullchallenge does not provide the same level of security as a challengethat requires computational resources to generate a response or achallenge that requires a human response, but since many spammers do notprovide resources to accept reply messages at all of the addresses theyuse, the null challenge helps to classify many messages.

Other variations of the architecture and processes described are used inother embodiments. For example, it may not be desirable to send achallenge directly from the mail receiver to the mail sender, since thatwould confirm receipt by the sender. Instead, the challenge may be sentfrom the auditor to the sender on behalf of the receiver. The auditormonitors the response and sends the report including results orinstructions to the mail receiver.

Improved registration of a sender of electronic messages has beendescribed. The improved registration system enables mail senders torespond to challenges efficiently while deterring spammers. Registrationmay be accomplished by various techniques, including challenge andresponse. In some embodiments, the challenge includes a machineanswerable question that requires a nontrivial amount of resource toanswer The need for a human response in all cases is eliminated, whilestill making it impractical for a spammer to respond to a large numberof challenges. The difficulty of the challenge may be adjusted to anappropriate level to deter spammers but not require too many resourcesfrom a legitimate sender. The challenge may alternatively include ahuman answerable question. An auditor is integrated into thechallenge/response system that simplifies the response process. Theauditor may also provide additional analysis and reporting to the mailreceiver, and may help avoid having the mail receiver interact directlywith the mail sender. In some embodiments, the challenge requires onlythat it be accepted by the mail sender and that an error message not begenerated in response to the challenge.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing both the process and apparatus of the present invention.Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalents of the appended claims.

What is claimed is:
 1. A method of registering an electronic-mail(e-mail) sender for the purpose of sending e-mail messages to an e-mailrecipient, the method comprising: receiving an e-mail sent by the e-mailsender to the e-mail recipient; executing instructions stored in memory,wherein execution of the instructions by a processor of an independentthird-party auditor generates a challenge message in response to ane-mail message received by the e-mail recipient, wherein the challengemessage does not invite a response to the challenge message from thee-mail sender; sending the challenge message to the e-mail sender;receiving an error message associated with the e-mail sender within apredetermined period of time after the sending of the challenge message,the error message suggesting that the e-mail sender is not a valide-mail address where mail can be sent, wherein the error message isevaluated by the independent third-party auditor; and sending a reportgenerated by the independent third-party auditor to the e-mailrecipient, the report indicating receipt of the error message, whereinthe e-mail recipient classifies e-mail messages from the e-mail senderbased on the report.
 2. The method of claim 1, wherein the e-mailrecipient also processes registration of the e-mail sender based on thereport.
 3. The method of claim 2, wherein the e-mail recipient does notregister the e-mail sender based on the report.
 4. The method of claim1, wherein the report indicates that the e-mail sender is a spammer. 5.The method of claim 1, wherein the e-mail recipient classifies e-mailmessages from the e-mail sender as spam messages based on the report. 6.The method of claim 1, wherein the report includes instructions to thee-mail recipient.
 7. The method of claim 1, wherein the challengemessage is a reply to the e-mail sent by the e-mail sender to the e-mailrecipient, the reply acknowledging receipt of the email.
 8. The methodof claim 1, wherein the error message includes at least one of an e-mailbounce message or an e-mail non-delivery message.
 9. A system ofregistering an electronic-mail (e-mail) sender for the purpose ofsending e-mail messages to an e-mail recipient, the system comprising:an independent third-party auditor server that executes instructionsstored in memory, wherein execution of the instructions by a processorof the independent third-party auditor generates a challenge message inresponse to an e-mail message received by an e-mail recipient, thee-mail sent by an e-mail sender to the e-mail recipient, wherein thechallenge message does not invite a response to the challenge messagefrom the e-mail sender; and a network interface coupled to theindependent third-party auditor server that receives the e-mail sent bythe e-mail sender to the e-mail recipient, wherein the networkinterface: sends the challenge message to the e-mail sender; receives anerror message associated with the e-mail sender within a predeterminedperiod of time after the sending of the challenge message, the errormessage suggesting that the e-mail sender is not a valid e-mail addresswhere mail can be sent, wherein the error message is evaluated by theindependent third-party auditor server; and sends a report generated bythe independent third-party auditor server to the e-mail recipient, thereport indicating receipt of the error message, wherein the e-mailrecipient processes registration of the e-mail sender based on thereport.
 10. The system of claim 9, wherein the e-mail recipient does notregister the e-mail sender based on the report.
 11. The system of claim9, wherein the e-mail recipient further classifies e-mail messages fromthe e-mail sender based on the report.
 12. The system of claim 11,wherein the e-mail recipient classifies e-mail messages from the e-mailsender as spam messages based on the report.
 13. The system of claim 9,wherein the report indicates that the e-mail sender is a spammer. 14.The system of claim 9, wherein the report includes instructions to thee-mail recipient.
 15. The system of claim 9, wherein the challengemessage is a reply to the e-mail sent by the e-mail sender to the e-mailrecipient, the reply acknowledging receipt of the email.
 16. The systemof claim 9, wherein the error message includes at least one of an e-mailbounce message or an e-mail non-delivery message.
 17. A non-transitorycomputer readable storage medium having embodied thereon a program,wherein the program is executable by a processor to perform a method ofregistering an electronic-mail (e-mail) sender for the purpose ofsending e-mail messages to an e-mail recipient, the method comprising:receiving an e-mail sent by the e-mail sender to the e-mail recipient;generating a challenge message, using an independent third-partyauditor, in response to an e-mail message received by the e-mailrecipient, wherein the challenge message does not invite a response tothe challenge message from the e-mail sender; sending the challengemessage to the e-mail sender; receiving an error message associated withthe e-mail sender within a predetermined period of time after thesending of the challenge message, the error message suggesting that thee-mail sender is not a valid e-mail address where mail can be sent,wherein the error message is evaluated by the independent third-partyauditor; and sending a report generated by the independent third-partyauditor to the e-mail recipient, the report indicating receipt of theerror message, wherein the e-mail recipient processes registration ofthe e-mail sender based on the report.
 18. The non-transitory computerreadable storage medium of claim 17, wherein the e-mail recipient doesnot register the e-mail sender based on the report.
 19. Thenon-transitory computer readable storage medium of claim 17, wherein thee-mail recipient further classifies e-mail messages from the e-mailsender based on the report.
 20. The non-transitory computer readablestorage medium of claim 17, wherein the challenge message is a reply tothe e-mail sent by the e-mail sender to the e-mail recipient, the replyacknowledging receipt of the email.